(1) Field of the Invention
The present invention relates generally to a process for the countercurrent, reactive distillation, esterification of organic di- and tri-acids with lower alcohols (C1 to C8) with the available acid groups esterified. Specifically, the present invention relates to citric acid tri-esterification using reactive distillation with the alcohols, particularly ethanol.
(2) Description of the Related Art
There has been considerable interest in the development and use of biorenewable-based chemicals. This interest is fueled by ever-rising global crude oil prices, the increasing desirability to reduce dependence on petroleum-based industries, and potential environmental benefits. Bio-based chemicals are environmentally friendly because they often degrade into benign products, making them prime candidates for replacement of petroleum-based products. The esters of bio-based organic acids fall into the category of benign or “green” solvents and are promising replacements for halogenated petroleum-based solvents in a wide variety of uses.
Citric acid can be esterified with alcohols such as ethanol and n-butanol to yield triethyl citrate and tri-n-butyl citrate, respectively. Both tri-ethyl citrate and tri-n-butyl citrate are used as non-toxic plasticizers in toys, medical products (e.g. as enteric coatings for controlled release drug delivery systems), printing ink coatings, cosmetics, etc. These plasticizers are suitable as food additives such as whipping agents for dried egg whites, food flavorings, or food packaging materials. Citrate esters rapidly metabolize in the body via liver and blood serum enzymes to liberate the citrate ion which is disposed of through the usual biochemical pathways.
Tri-ethyl citrate is a water soluble plasticizer while tri-butyl citrate is a water insoluble plasticizer. The global market for citrate plasticizers has been estimated at around 11 billion pounds per year and according to 2003 statistical data, the U.S. share of this market is 2.4 billion pounds.
Citric acid is a tri-basic acid. Upon esterification with an alcohol such as ethanol or n-butanol, it forms, through a series of reactions, a mixture of mono-alkyl, di-alkyl and tri-alkyl citrates as shown in FIG. 1.
The esterification of citric acid is an equilibrium-limited process. Such processes, when carried out in conventional reactors, suffer from limited conversion. To overcome this equilibrium limitation, continuous removal of one of the products of the reaction mixture is essential in order to drive the reaction to completion. One technique available for doing so is through the use of continuous reactive distillation.
There are other di- and tri-organic acids. These include succinic acid, maleic acid, glutaric acid, adipic acid, malonic acid and pimelic acid which are diacids. It also applies to other tri-acids which need to be esterified.
Reactive Distillation
Reactive distillation (RD) is a multifunctional process where chemical reaction and distillation occur simultaneously in a single vessel. The numerous advantages arising from the synergistic interaction of unit operations in a single unit over the conventional, sequential operation of a reactor followed by a distillation column include overcoming limitations of some equilibrium-limited chemical reactions (as in methyl acetate and MTBE production), improved selectivity by removal of the products from the reaction zone, thereby limiting by-product formation, “reacting away” some of the azeotropes present in conventional distillation processes, and utilizing the exothermic heat of reaction for vaporization and avoidance of hot spot formation. These potential advantages of reactive distillation lead to lower capital and operating costs as well as reduced environmental impact. The major shortcomings of reactive distillation are requirements for much greater complexity in design and process control, often involving multiple steady states, and high process non-linearity due to the strong interaction between process variables. The operating window of reactive distillation processes has to be compatible with the thermal stability of the catalyst in order to ensure a long catalyst life since frequent catalyst change over is an expensive proposition. Reactive distillation requires a significantly greater research effort, in both experimental and in mathematical modeling, in order to develop a feasible process.
Reactive distillation was first reported by Backhaus (Backhaus A. A. Continuous Process for the manufacture of Esters. U.S. Pat. No. 1,400,849 (1921)). The subject has witnessed an explosion of interest in the last ten years since its commercial application to MTBE and methyl acetate production. Reviews on reactive distillation are available from Sharma and Mahajani (Sharma, M. M., Mahajani, S. M. Industrial Application of Reactive Distillation in Reactive Distillation Edited by Sundmacher K. and Kienle A., Wiley VCH, Germany, 1 (2003)); Mahajani and Chopade (Mahajani, S. M.; Chopade, S. P. Reactive Distillation: Process of Commercial Importance. Encyclopedia of Separation Science Edited by Wilson I. D., Edlard T. R., Poole C. A. and M. Cooke, Academic Press, London UK, 4075 (2001)), Taylor and Krishna (Taylor, R., et al., Modelling Reactive Distillation. Chem. Eng. Sci., 55, 5183 (2000); and Modelling of Homogeneous and heterogeneous Reactive Distillation Processes, Chapter 9 in “Reactive Distillation” Editors: K. Sundmacher, A. Kienle, pp. 217-240, Wiley-VCH, Mannheim (2003) and Hiwale et al (Hiwale, R. S., et al., Industrial Application of Reactive Distillation: Recent Trends. Int. J. of Chem. React. Eng., 2, Review R1 (2004)). Reactive distillation in the presence of solid acid catalyst, also commonly referred to as catalytic distillation, has come a long way since its inception by Spes (Spes, H., Katalytische Reaktionen in Ionenaustaucherkolonnen unter Verschiebung des chemische Geleichgewichts. Chemiker Atg/Chemische Apparatur, 90, 443-446 (1966)). Industrially applied reactive distillation processes are rather limited in number currently, especially in comparison to potential applications (Stankiewicz and Moulijn, 2000).
The degree of complexity in reactive distillation processes increases with use of solid catalysts, but the development of modern structured column packings makes the use of such catalysts viable. Katapak-S structured packing (Sulzer), used in this invention, is a well-known and highly versatile packing consisting of vertically-oriented enclosures filled with catalyst particles. Fluid dynamic properties and use of Katapak-S in reactive distillation systems have been described in the open literature by Moritz and Hasse (Chem. Eng. Sci. 54, 1367-1374 (1999), Hanika et al (Chem. Eng. Sci. 54, 5205-5209 (1999), Van Baten et al (Catalysis Today, 69, 371-377 (2001), Gotze et al (Catalysis Today., 69, 201-208 (2001), Van Baten and Krishna (Catalysis Today, 69, 371-377 (2001), Smejkal et al (Chem. Eng. Sci., 56, 365-370 (2001), Ratheesh and Kannan (Chem. Eng. Ji., 104, 45-54 (2004), Kolodziej et al (Chem. Eng. Proc., 43, 457-464 (2004), Schmitt et al (Chem. Eng. Proc., 43, 397-409 (2004) and Schmitt et al (Chem. Eng. Proc. 44, 677-685 (2005).
Citric Acid Esterification Via Reactive Distillation
Synthesis of organic acid esters by reactive distillation is well established, but in most applications the ester has either the highest volatility of the reagents present (e.g., methyl acetate) or the lowest volatility, with water as the most volatile component (Schmitt et al., Chem. Eng. Proc. 43, 397-409 (2004)). In these cases, recovery of 100% pure ester is straightforward via optimization of column operating conditions. Tri-ethyl citrate production via reactive distillation does not fit into either of these categories. Formation of tri-ethyl citrate proceeds through a series of reactions yielding mono- and di-ethyl citrate as intermediate products. Since tri-ethyl citrate has a volatility that is lower than ethanol and water but higher than citric acid, mono- and di-ethyl citrates (those are essentially non-volatile). Therefore, it is only possible to isolate the pure product if complete conversion of citric acid and the intermediate products mono-ethyl citrate and di-ethyl citrate are achieved within the reactive distillation column. The primary challenge is therefore to achieve sufficiently rapid esterification kinetics so as to ensure complete conversion to the desired product triethyl citrate. Example of previous experimental work on similar esterification systems has been described by Bock et al (Chem. Eng. Prog. 36, 101-109 (1997)) for the synthesis of isopropyl myristate, and in Applicants' parent application Ser. No. 10/894,307, filed Jul. 19, 2004 for synthesis of ethyl lactate which is incorporated herein in its entirety. In both of these systems excess ethanol is used, which distills along with water as the top product. Omota et al (Chem. Eng. Sci., 58, 3159-3174 (2003)) have described a reactive distillation system for synthesis of fatty esters where an immiscible two-phase water-alcohol mixture distills as the top product.
Prior information on the kinetics of citric acid esterification with ethanol or n-butanol is confined to mainly the Chinese and German patent literature. Joerg et al. (Application WO 2003008369 A1 (2003)) describes a process for synthesis of tri-ethyl citrate by a three stage batch process. Tao (Tao, X., Huazue Shijie, 39(6), 302-304 (1998)) discusses the synthesis of tri-ethyl citrate in the presence of p-toluenesulfonic acid as catalyst and removal of the formed water. Frappier et al (WO 9851657 (1998)) discusses a process for synthesis of tri-ethyl citrate from citric acid containing fermentation broths. Some of the recent references on synthesis of tri-butyl citrate have been described by Nong (Nong, L., Synthesis of Tributyl Citrate with Aluminum Phosphotungstate Supported on activated Carbon. Jingxi Huagong Zhongjianti, 34, 50-52 (2004)) using an aluminum phosphotungstate supported catalyst, Shi et al (Synthesis of tributyl citrate catalyzed by solid superacid S2O82−/TiO2/SiO2. Yingyong Huagong Keji, 33, 41-43 (2004)) using a solid super acid catalyst, Zheng et al (Synthesis of tributyl citrate catalyzed by tetrabutyl titanate. 34, 28-30 (2004)) using tetrabutyl titanate, Deng et al. (The synthesis of tributyl citrate catalyzed by sodium hydrogen sulfate. Jingxi Huagong Zhongjianti, 33, 49-50 (2003)) using sodium hydrogen sulfate, Song et al. (Catalytic synthesis of tri-n-butyl citrate with aluminophosphate solid acid catalyst. 11, 6-8 (2003)) using an aluminophosphate solid acid catalyst, Liu et al (Catalytic synthesis of tri-butyl citrate with dealuminated USY zeolite. 11, 175-177 (2003)) using a dealuminated USY Zeolite, Meng et al. (Synthesis of tributyl citrate catalyzed by the nanosolid superacid SO42−/Fe2O3. Hebei Shifan Daxue Xuebao, Ziran Kexueban. 27, 64-66 (2003)) using a nanosolid superacid SO42−/Fe2O3, Zhao et al. (Synthesis of tributyl citrate catalyzed by complex solid superacid WO3/TiO2/SO42−. Huagong Keji. 10(5), 11-13 (2002)) using WO3/TiO2/SO42− super acid catalyst, Meng et al. (Synthesis of tributyl citrate with p-toluene sulfonic acid catalyst. Hecheng Huaxue Ji Suliao, 19(2), 16-18 (2002)) using p-toluenesulfonic acid catalyst and Fu et al. (Synthesis of tributyl citrate catalyzed by SO42− modified zirconium cross-linked solid clay. Jingzi Huagong. 19(1), 28-31 (2002)) using SO42− modified zirconium crosslinked clay catalyst.
U.S. Pat. No. 5,008,046 to Bremus et al. describes a reactive distillation process using a column with plate type column with an acid catalyst under pressure. U.S. Pat. No. 5,536,856 to Harrison et al. describes a similar column wherein resin acid catalyst particles were supported by a tray on the column. The use of columns with trays is quite expensive.
While the related art teaches organic acid esterification processes, there still exists a need for improved continuous processes for di- and tri-organic acid, particularly citric acid esterification.